Submodules vs subtrees comparison in Git - When to Use Which

The Big Idea

Discover how to stop juggling code copies and start managing projects like a pro!

The Scenario

Imagine you have a big project that uses code from other projects. You try to copy and paste those codes manually into your project folder every time they update.

The Problem

This manual copying is slow and confusing. You might forget to update some parts or mix up versions. It's hard to keep track of changes and share your project with others without errors.

The Solution

Using submodules or subtrees in Git helps you link other projects inside your main project cleanly. They keep the code connected and updated without messy copying, making teamwork and updates easier.

Before vs After

✗ Before

Copy code folder from external project
Paste into your project
Repeat on every update

✓ After

git submodule add <repo-url>
OR
git subtree add --prefix=folder <repo-url> main --squash

What It Enables

You can manage external code inside your project smoothly, track updates clearly, and collaborate without confusion.

Real Life Example

A web app uses a shared UI library maintained by another team. Using submodules or subtrees, the app always gets the latest UI updates without manual copying.

Key Takeaways

Manual copying of external code is error-prone and slow.

Submodules link external repos as separate parts, keeping history separate.

Subtrees merge external code into your repo, simplifying management.

Practice

1. What is the main difference between Git submodules and Git subtrees?

easy

A. Submodules and subtrees are identical in how they manage external repositories.

B. Submodules copy external repositories; subtrees link them separately.

C. Submodules merge external repositories automatically; subtrees require manual merging.

D. Submodules link external repositories separately; subtrees copy them into your project.

Solution

1. Step 1: Understand submodules behavior

   Submodules keep external repositories linked but separate from the main project folder.

2. Step 2: Understand subtrees behavior

   Subtrees copy the external repository content directly into your project folder, making it part of your codebase.

3. Final Answer:

   Submodules link external repositories separately; subtrees copy them into your project. -> Option D

4. Quick Check:

   Submodules = separate links, Subtrees = copied content [OK]

Hint: Remember: submodules link, subtrees copy [OK]

Common Mistakes:

• Confusing which method copies code versus links it
• Thinking submodules merge code automatically
• Assuming subtrees keep repos separate

2. Which Git command correctly adds a submodule to your repository?

easy

A. git subtree add --prefix=folder_name repo_url branch

B. git submodule add repo_url folder_name

C. git add submodule repo_url folder_name

D. git clone --submodule repo_url folder_name

Solution

1. Step 1: Identify the correct syntax for adding a submodule

   The correct command is 'git submodule add' followed by the repository URL and the folder name.

2. Step 2: Verify other options

   git subtree add --prefix=folder_name repo_url branch is for subtrees, not submodules. git add submodule repo_url folder_name and git clone --submodule repo_url folder_name are invalid Git commands for adding submodules.

3. Final Answer:

   git submodule add repo_url folder_name -> Option B

4. Quick Check:

   Submodule add uses 'git submodule add' [OK]

Hint: Submodule add always starts with 'git submodule add' [OK]

Common Mistakes:

• Using 'git subtree add' for submodules
• Trying to add submodules with 'git add'
• Confusing clone options with submodule commands

3. Given the commands below, what will be the result after cloning a repository with a submodule?

git clone https://example.com/main-repo.git
cd main-repo
git submodule update --init --recursive

medium

A. Only the main repository is cloned; submodules remain empty folders.

B. The submodules are cloned but not initialized.

C. The main repository and all submodules are cloned and initialized.

D. The clone fails because submodules require separate cloning.

Solution

1. Step 1: Understand cloning with submodules

   Cloning the main repo alone does not fetch submodule content; submodules are empty initially.

2. Step 2: Use 'git submodule update --init --recursive'

   This command fetches and initializes all submodules recursively, filling their folders with content.

3. Final Answer:

   The main repository and all submodules are cloned and initialized. -> Option C

4. Quick Check:

   Submodule update with init fetches submodules [OK]

Hint: Run 'git submodule update --init --recursive' after clone [OK]

Common Mistakes:

• Assuming submodules clone automatically with main repo
• Skipping the submodule update command
• Confusing initialization with cloning

4. You added a subtree with git subtree add --prefix=lib https://example.com/lib.git main but later changes in the subtree are not reflected in your main repo. What is the likely cause?

medium

A. You forgot to pull subtree updates with git subtree pull.

B. Subtrees automatically sync changes; no action needed.

C. You must use git submodule update to sync subtrees.

D. Subtrees cannot be updated once added.

Solution

1. Step 1: Understand subtree update process

   Subtrees copy code but do not auto-sync; you must manually pull updates.

2. Step 2: Identify correct command to update subtree

   The command 'git subtree pull' fetches and merges changes from the external repo into your subtree folder.

3. Final Answer:

   You forgot to pull subtree updates with git subtree pull. -> Option A

4. Quick Check:

   Subtrees require manual pull to update [OK]

Hint: Use 'git subtree pull' to update subtree changes [OK]

Common Mistakes:

• Expecting automatic subtree sync
• Using submodule commands for subtrees
• Thinking subtrees are read-only

5. You want to include a third-party library in your project. You want to keep it separate for easy updates but also want your team to clone the project with one command without extra steps. Which approach is best?

hard

A. Use Git subtrees to copy the library into your project for simpler cloning.

B. Use Git submodules and instruct the team to run extra commands after cloning.

C. Manually copy the library files without Git integration.

D. Use Git submodules but disable recursive cloning.

Solution

1. Step 1: Analyze submodules pros and cons

   Submodules keep code separate but require extra commands after cloning to initialize submodules.

2. Step 2: Analyze subtrees pros and cons

   Subtrees copy the library into the project, so cloning is simple with one command and no extra steps.

3. Final Answer:

   Use Git subtrees to copy the library into your project for simpler cloning. -> Option A

4. Quick Check:

   Subtrees simplify cloning by embedding code [OK]

Hint: Subtrees = easy clone; submodules = extra steps [OK]

Common Mistakes:

• Choosing submodules despite extra clone steps
• Ignoring subtree benefits for team convenience
• Thinking manual copy is better than Git methods